Add critical_temperature argument to VaporPressure constructor

[g-bauer]

This PR adds a new optional argument to the constructors of VaporPressure and DataSet.vapor_pressure, i.e. the critical temperature.

• If provided, it is used as initial temperature when calculating the critical point in the predict method.

• If not provided, the maximum temperature of the input temperatures is used.

• If the calculation of the critical point within predict fails (using either the provided or maximum temperature), another call to the method using the default temperatures is used.

• Did some cleanup of the documentation and added eos.estimator to the documentation.

[prehner]

Excellent stuff! I tried it on a larger dataset and it works as intended. Nice compromise between robustness and flexibility.